The present invention relates to making needled textile structures for use in the manufacture of protective parts used at high temperatures, structural parts of rocket engines, or indeed very high performance brake disks for aviation or for terrestrial vehicles.
Brake disks need to withstand braking that creates particularly large shear forces. This phenomenon is accentuated in aircraft because of the large stresses applied to brake disks.
In order to withstand these shear forces that give rise to a delaminating effect, disks must be manufactured in such a manner as to minimize structural non-uniformities. A non-uniform disk has localized zones in which stress characteristics are heterogeneous, thereby considerably increasing the risk of tearing.
Conventionally, brake disks are made from a reinforcing textile structure made up of a plurality of superposed layers that are needled together by a set of barbed needles penetrating in a z direction, i.e. transversely relative to the layers. After being cut to size, the textile structure is carbonized, is then densified using a matrix-forming material, and is finally subjected to optional heat treatment. The layers are superposed on a support. A downward movement step is generally imparted to the support as the superposed layers build up, and needling is performed in the various layers. The mechanical characteristics of the final product obtained in this way depend very greatly on the real needling density used in the textile reinforcing structure. The term xe2x80x9crealxe2x80x9d needling density is a function of the number of needle barbs per cubic centimeter (cm3) seen by an elementary volume of the textile structure, and therefore includes the needling density per unit area, the extend of z penetration, the size of the downward displacement step, and the functional characteristics of the needles.
Present-day needling methods make it difficult to obtain the desired perfect uniformity, even though some methods do provide good results, in particular by acting on the size of the downward step. Mention can be made of U.S. Pat. No. 4 790 052 which proposes that the distance between the needles and the layer support be increased for each superposed layer by a distance that is equal to the thickness of a needled layer. Mention can also be made of European patent No. 0 736 115 which seeks to obtain constant thickness for the various superposed layers by adopting displacement steps for the layer support that are of a size that reduces in compliance with a predetermined relationship.
The imperfection of those methods comes from the fact that the size of the downward displacement step for the layer support is generally calculated beforehand on theoretical grounds, in particular as a function of the number of layers that are to form the resulting textile structure, and no account is taken of the real penetration depth of the needles. Unfortunately, it is essential to know this parameter in order to guarantee uniform needling density which is a requirement for obtaining a final textile structure presenting good uniformity. In addition, the greater the thickness of the textile structure, the greater the margin of error concerning knowledge about penetration depth.
European patent application No. 0 695 823 seeks to improve knowledge of needle penetration depth by means of feeler rollers that measure the position of the top surface of the textile structure between needling operations and that are disposed beside the working zone for the needles.
Nevertheless, such a solution turns out to be unsatisfactory since under the action of needling forces, the textile structure is compressed in a way that the measurement performed does not detect. This failure to take account of the deformation of the textile structure means that it is not possible to have exact knowledge concerning the real penetration depth of the needles.
The present invention thus proposes a needling machine and an associated method which mitigate this drawback by enabling the real penetration depth of needles in the textile structure for needling to be measured in such a manner as to take account of the deformation of the structure while performing the operations of needling the various layers that make it up.
This object is achieved by a machine for needling a textile structure made up of a plurality of superposed layers, the machine comprising a vertically movable needling table, a needling head having a determined number of barbed needles placed vertically above said needling table, and means for driving said needling head to impart vertical reciprocating motion thereto defining a low point of maximum needle penetration in said textile structure, the machine further comprising measuring means disposed in said needling head to measure the position of the top surface of said textile structure at the low point of maximum penetration of the needles.
Thus, by positioning the measuring means in the needle board it is possible to take account of the extent to which the textile structure is compressed under the effect of the needling forces, thereby making it possible to determine accurately the real penetration depth of the needles.
Preferably, said measuring means are disposed in a midplane of said needling head perpendicular to an advance direction of said textile structure.
In a preferred embodiment, said means for measuring the position of the top surface of said textile structure comprise an optical assembly for performing contactless measurements. Preferably, this comprises a broad beam type of laser emitter/receiver.
In an alternative embodiment, said measurement means can comprise a mechanical feeler for measuring by contact.
Advantageously, a sensor is provided, preferably of the inductive or optical type, to determine said low point of maximum needle penetration, and processor means are provided for controlling the vertical displacement of said needling table as a function of the position of the top surface of the textile structure as measured at said low point of maximum needle penetration by said measuring means.
The invention also provides a method of using the above-mentioned machine to make a textile structure, and to the textile structure obtained by the method. The position of the top surface of the textile structure is preferably measured by means of instantaneous measurements performed in real time over the entire length of the textile structure.